Explanation:
Many stars form in
clusters.
Galactic or
open star
clusters are relatively young
swarms of bright stars
born
together near the plane of our Milky Way Galaxy.
Separated by about a degree on the sky, two
nice examples are M46
(upper left) 5,400 light-years in the distance
and M47
(lower right) only 1,600 light-years away toward
the nautical constellation
Puppis.
Around 300 million years
young M46
contains a few hundred stars in a region about 30 light-years
across.
Aged 80 million years,
M47 is a
smaller but looser cluster
of about 50 stars spanning 10 light-years.
But
this portrait of stellar youth also contains
an ancient
interloper.
The small, colorful patch of glowing gas in
M46 is actually
the planetary nebulaNGC 2438 - the
final phase in the life of a sun-like star billions
of years old.
NGC
2438 is estimated to be only 3,000 light-years distant
and likely represents a foreground object, only by chance appearing
along our line of sight to youthful M46.